The present invention provides a method which achieves a simultaneous coulouring and straightening of the hair in a single step, utilizing a hair treatment composition comprising at least one carboxylic acid of formula (I) defined below and at least one direct dye: R&mdashCO&mdashCOOH&emsp&emspFormula (I) In another aspect, the invention is directed to the hair treatment composition and the use thereof for straightening and colouring the hair.
